1. Field of the Invention
The present invention relates to a technique for embedding digital-watermark information in an image.
2. Description of the Related Art
In recent years, digital data, such as text data, image data, and voice data, has been widely used, and it has become necessary to prevent unauthorized duplication of the data. Also, documents and images are often used in a printed form, and thus unauthorized duplication of the printed material must be prevented.
Digital-watermark embedding has been known as a technique for preventing unauthorized duplication. In this technique, original digital image data is modulated so that the data is imperceptible or difficult to perceive, and another piece of information is embedded in the data.
When digital-watermark information is embedded, it is very important to determine whether or not necessary amount of information can be embedded in original image data.
The following three elements are in tradeoff relationship: (1) quality of image data in which digital-watermark information is embedded; (2) robustness to attack (image editing and so forth) on the image data; and (3) the amount of digital-watermark information to be embedded in the original image data.
For example, if degradation of image quality caused by digital-watermark embedding is suppressed, robustness to attack is reduced or the amount of information to be embedded is reduced. If the amount of information to be embedded is increased, degradation of image quality caused by digital-watermark embedding becomes significant or robustness to attack is reduced. If robustness to attack is increased, degradation of image quality caused by digital-watermark embedding becomes significant or the amount of information to be embedded is reduced. The robustness to attack is referred to also as embedding strength.
As a technique for embedding digital-watermark information in a multivalued image, a method using redundancy of pixel density has been known. Also, as a technique for embedding digital-watermark information in a binary document image, a digital-watermark embedding method using the characteristic of the document image has been known. For example, Japanese Patent Laid-Open No. 9-186603 (U.S. Pat. No. 5,861,619) discloses a method for embedding digital-watermark information by changing the length of space between words. In such a method, digital-watermark information is represented by changing the length of space between words or letters, and 1-bit information (1 or 0) is allocated in accordance with the length of two spaces.
Also, “Digital Watermarking onto Japanese Documents by Seal Image”, written by Yasuhiro NAKAMURA and Kineo MATSUI (Information Processing Society of Japan Journal Vol. 38, No. 11, November 1997), discloses a technique of embedding digital-watermark information by rotating letters so as to change the inclination angle of the letters.
FIG. 24 shows an example of the above-described known art. For example, when a letter is rotated clockwise, “1” is embedded therein as shown in (1) in FIG. 24. When a letter is rotated counterclockwise, “0” is embedded therein as shown in (2) in FIG. 24. Watermark may be embedded in sequential letters, or every few letters, or in a letter at a predetermined position. In FIG. 24, the letter “C” is rotated clockwise and the letter “E” is rotated counterclockwise. Accordingly, information “10” is embedded in this case.
Now, a method for embedding digital-watermark information in a multivalued image will be described. Hereinafter, image data represents a monochrome multivalued image for clarity.
A binary data sequence is regarded as additional information Inf. The additional information Inf is information including some bits, each bit representing “0” or “1”. Then, digital-watermark information w is generated based on the additional information Inf. In the simplest method of generating the digital-watermark information w, an image is scanned by raster scanning and the additional information Inf is associated with the position of image data I. When a bit represents “0”, −1 is allocated, and when a bit represents “1”, +1 is allocated.
Then, the image data I and the digital-watermark information w are input, the digital-watermark information w is embedded in the image data I, and then image data I′ in which the digital-watermark information w is embedded is output.
For example, digital-watermark information embedding is performed in accordance with an equation: I′i,j=Ii,j(1+awi,j). Herein, I′i,j is image data in which digital-watermark information is embedded, Ii,j is image data before the digital-watermark information is embedded therein, wi,j is the digital-watermark information, i and j are parameters representing x and y coordinates of I and I′ and w, respectively, and a is a parameter specifying the embedding strength of the digital-watermark.
Now, the strength a will be described. For example, when the strength a is “0.01”, it means that about 1% of the element value of an original image is changed. By increasing the value of a, digital-watermark information which is robust to attack can be embedded. In that case, however, quality of the image is significantly degraded. On the other hand, by decreasing the value of a, the robustness to attack will be reduced. In that case, however, degradation of the image quality can be suppressed.
That is, by adequately setting the value of a, robustness to attack and the quality of water-marked image can be kept in balance. When digital-watermark information is embedded in a multivalued image, the amount of information which can be embedded therein is effectively specified based on the size of the original image.
On the other hand, when digital-watermark information is embedded in a document image, the amount of information which can be embedded is generally proportional to the number of letters. The documents may include newspapers, presentation material, and postcards, and the number of letters included in these documents is different from each other. Therefore, when digital-watermark information is embedded in a document image, it is difficult and is not preferable to estimate the amount of information which can be embedded therein in advance.
However, an efficient method of embedding digital-watermark information in a document image considering the above-described three elements has not been known. Also, an efficient method of embedding digital-watermark information in a multivalued image considering the above-described three elements has not been established.